In the Oman ophiolite, one of the best preserved and most studied ophiolites in the world, two distinct petrogenetic suites of gabbroic rocks from the layered gabbro sequence of the Wadi Haymiliyah ...section is established using trace element chemistry of Ca-rich clinopyroxenes. The earlier GB1 suite is characterized by plagioclase with lower An (Ca/(Ca
+
Na)) content and clinopyroxene with low large-ion lithophile elements (LILE) concentrations. The later DWGB2 suite contains plagioclase with rather high An content and clinopyroxene with high LILE. This difference in clinopyroxene chemistry can be extended to the extrusive rocks in this section: lower, (earlier) HV1 suite with low LILE clinopyroxene and upper (later) HV2 suite with high LILE clinopyroxene. Difference in LILE concentration of clinopyroxenes is essentially due to geochemical difference in parental magmas. The GB1/HV1 suites formed at fast-spreading MOR setting and DWGB2/HV2 suites at SSZ setting, supporting a model of transition from mid-oceanic ridge to supra-subduction zone settings of the Oman ophiolite. Our results indicate that geochemical signature of clinopyroxene is a very strong tool for identification of tectonic setting of ophiolites.
High‐Ca boninitic inclusions are found in primitive low‐K tholeiite from Mukoojima (Mukoo‐Jima), an islet in the Hahajima Island group, Bonin (Ogasawara) forearc, Japan. While Chichijima Island ...group, 50 km north of Hahajima Island group, is well known as a type locality of boninite, there has been no report of boninitic rocks from the Hahajima Island group. The high‐Ca boninitic inclusions are aphanitic and contain olivine, Ca‐rich clinopyroxene, plagioclase, chromian spinel, opaque minerals and dark brown glass. The mode of occurrence of the inclusions and host tholeiite under the microscope indicates mingling of these two magmas, suggesting intimate association in space and time of the boninite and primitive tholeiite magmas around the Hahajima Island group in Paleogene time. Primitive compositions and slightly different Sr and Nd isotopic ratios suggest that these two magmas are derived from two distinct mantle sources. These two mantle sources were present at the same time around the Hahajima Island group, southern Bonin forearc. The source of the high‐Ca boninite was higher in water content and/or shallower in depth compared to that of the primitive tholeiite.
The cooling rate of a geological material is generally calculated using two isotopic ages with different closure temperatures; however, the closure temperature is itself dependent on the cooling ...rate. Here we present a simple procedure for calculating a consistent set of closure temperatures and cooling rate using Dodson’s mathematical formula.
A Ni-Fe alloy (Ni3.8Fe0.1Co0.1) was found in olivine-poor gabbro recovered from the Integrated Ocean Drilling Program (IODP) Hole U1309D, Atlantis Massif, 30°N on the Mid-Atlantic Ridge. Ni-Fe alloys ...are one of the unique minerals formed during serpentinization; however, our finding establishes that Ni-Fe alloys are also formed in the gabbroic crust possibly associated with extremely reduced magmatic fluids. The occurrence of the Ni-Fe alloy suggests that oceanic lower-crustal gabbros as well as serpentinites may be potential reservoirs of abiogenic CH4.
Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and ...crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks--in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas--provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.
Program canceled for a disaster Program canceled for the 2018 Hokkaido Eastern Iburi earthquake. However, This abstract is quotable and viewable on PDF.
We measured 87Sr/86Sr and 143Nd/144Nd ratios and carried out analytical test of Sr and Nd isotopic standards for the seven GSJ rock reference samples (hereafter GSJ-RRSs); JA-1, JB-1a, JB-2, JB-3, ...JG-1a, JGb-1 and JR-1. Our mean values of five separate analyses for the GSJ-RRSs are summarized in Table 1, in agreement with the ranges of previously published data. Our mean values for 87Sr/86Sr ratios of JB-2 and JGb-1 and 143Nd/144Nd ratio of JR-1, however, show a slightly large variation as compared with the other JGS-RRSs. Further data accumulation with specification of split/position number is required to evaluate homogeneity in Sr and Nd isotopic compositions.
Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by ...itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle–ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective associations with ductile and cataclastic deformation, and an overall decrease downhole with hydrothermal alteration generally ≤5% in the bottom kilometer. Hole 735B provides evidence for a strongly heterogeneous lower ocean crust, and for the inherent interplay of deformation, alteration and igneous processes at slow-spreading ridges. It is strikingly different from gabbros sampled from fast-spreading ridges and at most well-described ophiolite complexes. We attribute this to the remarkable diversity of tectonic environments where crustal accretion occurs in the oceans and to the low probability of a section of old slow-spread crust formed near a major large-offset transform being emplaced on-land compared to sections of young crust from small ocean basins.
Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and ...crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks-in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas-provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt. PUBLICATION ABSTRACT
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